The present disclosure relates to a semiconductor device, a motor driving system, and a motor control program and can be suitably used for control of a sensor-less brushless DC motor (also referred to as a permanent magnet synchronous motor), for example.
In sensor-less control of a brushless DC motor, a back electromotive force (BEMF) generated in a stator winding of a non-conduction phase because of rotation of a rotor of the motor is detected, so that a relative position of a magnetic pole pair of the rotor with respect to the stator winding is estimated.
Speed control of such a sensor-less brushless DC motor generally uses PWM (Pulse Width Modulation) control. When the motor is rotated at a high speed in PWM control, a duty ratio (also referred to as a rate of conduction) becomes large. When the motor is rotated at a low speed, the duty ratio becomes low.
However, when the duty ratio is made extremely low to achieve low-speed rotation, a duration of the back electromotive force generated in the stator winding in a non-conduction time period also becomes extremely short, thus making detection of the back electromotive force difficult. This is because, as an on-time becomes shorter than a delay time of a control signal of a motor control circuit, setting of a timing of reading the back electromotive force becomes difficult.
Japanese Unexamined Patent Application Publication No. 2012-165603 deals with the above problem. Specifically, a motor driving device described in this document makes a PWM frequency smaller in a stepwise manner with reduction of a rotation speed of a motor. This driving makes an on-time longer in a case of low-speed rotation, thus enabling reading of a back electromotive force.